In a video camera, a position of an object lens is varied to image an object, at various distances, onto a sensor. The position of the object lens is referred to as the “focus point.” The image on the sensor can be processed to measure contrast. The contrast measurement is referred to as a “focus value.” The focus value can be generated by deriving a high pass signal of the image formed on the sensor. The maximum focus value occurs when the object lens is at a position where a clear and sharp image is produced on the sensor. The position where a clear and sharp image is produced on the sensor is called the “correctly-focused point.”
Referring to FIG. 1, a diagram of an example focus value curve 10 is shown. The focus value curve 10 illustrates a relationship between focus value and focus point for an object at a predefined distance. The focus value appears as a “mountain-like”curve over the entire range of the focus point. The focus value reaches a maximum value when the position of the object lens corresponds to the correctly-focused point.
Automatic focus (or auto-focus) is a technique for finding and positioning the object lens on the correctly-focused point. A conventional auto-focus technique is referred to as a “full search” method. The full search method searches the entire focus point range (or region) to find the focus point corresponding to the maximum focus value (i.e., the correctly-focused point), and then positions the object lens at that point.
The full search auto-focus method can always find the correctly-focused point. However, the full search auto-focus method has several drawbacks. Firstly, to search the entire focus point range, the full search method consumes a large amount of time before finally settling at the correctly-focused point. Secondly, during the full search, the object lens moves first towards the correctly-focused point, then moves away from the correctly-focused point, and finally moves back to the correctly-focused point after searching the entire focus point region. Visually, the object appears to come into focus, go out of focus and return into focus after a while. The change in object clarity (focus) produces a poor user experience because the user expects the object to not move out of focus (i.e., become blurred) once the object is in focus (i.e., clear).
It would be desirable to have an auto-focus technique that improves focus speed and accuracy.